1. Field of the Invention
The present invention relates to an image forming device of the electrophotographic type such as a copier, facsimile machine, printer or printing machine, and a control method therefor, and particularly relates to a fixing device and a control method therefor in which fixing properties are stable and curl is not produced in the transfer paper after fixing, by performing temperature correction control of fixing of the toner image on the transfer paper constituting a sheet-like recording medium, using pressure or heat.
2. Description of the Related Art
Image forming device of this type have become very common in recent years. In particular, improvements in for example image quality, energy saving, and convenience are continually being demanded. Of these, since the energy consumption of the fixing device that is mounted in the image forming device represents 50% or more, efforts are being made to improve energy saving of the fixing device. Efforts are therefore being made to shorten the starting up time or recovery time by reducing the thickness and decreasing the amount of heat used by the fixing members constituting the fixing device.
As a fixing device, a construction of the fixing roller type is known, in which a pair of rollers are arranged facing each other, one of the rollers being used as a heating roller while the other roller is used as a pressurizing roller for applying pressure to the sheet-like recording medium, such as transfer paper. In this construction, fixing is effected by melt bonding of the unfixed image by heat from the heating roller, while the recording medium is fed while being gripped in the nip between the heating roller and pressurizing roller.
Apart from this fixing roller type construction, a fixing belt type construction is also known, comprising an assembly of rollers and a belt. In this construction, instead of the heating roller, a belt is employed that is passed over a pair of rollers, with a pressurizing roller being arranged facing one of these rollers. Of this pair of rollers, a heat source is provided for heating from the inside face of the belt at the roller that drives the belt in co-operation with the roller on the side facing the pressurizing roller, and a heat source is also provided at the pressurizing roller for heating the outside face of the belt. The belt is of smaller volume than the rollers and its heat capacity is small, so it can be raised in temperature in a short time, so the advantage is obtained that initial elevation of temperature at start-up can be achieved more rapidly than in the case of a construction using only a heating roller and pressurizing roller as described above. Furthermore, elevation of temperature at both the outside and inside of the belt is speeded up by providing a heat source at the pressurizing roller. It should also be noted that a double layer construction is known, in which, when aluminum, which is of high thermal conductivity, is used to construct the rollers in the case of a belt construction, in the case where stainless-steel is employed as the substrate that effects contact at the outer surface of the rollers, a belt body comprising a releasing layer made of silicone rubber or fluorine-based resin is arranged at the outer surface thereof.
However, as described above, although, with the conventional fixing device, rapid start-up can be achieved, since it is possible to rapidly heat specified portions of the fixing member by reducing the or heat capacity of the fixing member, when the entire fixing device or the entire image forming device becomes warmed up, in the case of several printing cycles or where the time for which the printing action is repeated is prolonged (hereinbelow this will simply referred to as “over time”), the amount of heat that is applied to the toner or transfer paper becomes excessive, so problems arise in that abnormal images, poor feeding or poor stacking due to curling of the transfer paper tend to occur. For this reason, consideration has been given to predicting the temperature increase over time and setting the target control temperature of the fixing device lower from the initial period. However this leads to problems of generation of abnormal images due to insufficient heating in the initial period on starting up or after recovery of operation of the fixing device.
Furthermore, in addition to the above problems, the transfer paper constituting the recording medium prior to transfer is not at a fixed temperature but is affected by the storage environment. Consequently, the amount of heat that is required when introducing the transfer paper to the fixing device after transfer may change, depending on the storage environment temperature of the transfer paper prior to transfer. In addition to temperature control within the fixing device, the fixing temperature must therefore be controlled using a plurality of detection means that detect the temperature and humidity of the external air wherein the image forming device is held and the temperature and humidity within the image forming device.
As prior art relating to control of fixing temperature, an image forming device as disclosed in Laid-open Japanese Patent Application No. H. 10-39672 is known. In this image forming device, a temperature sensor is arranged in a position where it is unlikely to be affected by the heat generated by the fixing device in the device interior, but the internal temperature of the image forming device changes depending on the job, so the environmental temperature cannot be precisely detected. Consequently, the value of this sensor is not used after job commencement; rather, the fixing temperature is corrected in accordance with environmental temperature detection during standby. It should be noted that this publication does not disclose the specific location of the sensor.
Furthermore, Laid-open Japanese Patent Application No. 2004-212968 discloses an image forming device wherein elevation of the temperature within the image forming device is suppressed by the provision of temperature detection means that detects the temperature of the fixing belt and a control unit that performs control of the image forming processing in accordance with the detected temperature. In addition, Japanese Patent No. 3425040 discloses an image forming device that provides excellent fixing properties by preventing the existence of a large difference between the standby control temperature and operating control temperature, even where fluctuation of the environmental temperature during standby has taken place, by correcting the fixing control temperature during standby, in accordance with the environmental temperature.
However, in these items of prior art, fixing temperature correction in accordance with environment detection is not performed over time. Consequently, even in cases where the temperature of the transfer paper after transfer is not affected by the storage environment, since the periphery of the fixing device and the feed path rise in temperature over time, when the transfer paper enters the fixing device, the amount of heat applied to the transfer paper after transfer becomes excessive, leading to problems of curling of the transfer paper after fixing. When the paper curls, alignment of the paper on discharging is lost, giving rise to problems of the transfer paper escaping from the binding by the finisher or being displaced in this binding.